The present application relates generally to the growth and structure of non-c-axis oriented ferroelectric materials. In particular, the present invention relates to anisotropic perovskite materials grown using a template layer formed on a buffered silicon substrate.
Ferroelectric perovskite materials are presently being studied as an alternative to conventional magnetic materials for use in digital memory systems. In particular, non-volatile memory devices are important computer components due to their ability to retain information after power has been removed or interrupted. Non-volatile memories that use ferroelectric thin films have been termed ferroelectric random access memories, or FRAMs or FERAMs. The structure of a FRAM cell can be similar to that of a conventional dynamic random access memory (DRAM) cell, but with the ferroelectric film replacing the dielectric material in the capacitor. Binary digital information is stored in the polarization states of the ferroelectric film.
Accordingly, numerous investigations of polycrystalline, bismuth-containing layered (i.e., anisotropic) perovskite thin films, such as SrBi2Ta2O9 (SBT) and SrBi2Nb2O9 (SBN) thin films, and La-substituted Bi4Ti3O12 such as Bi3.25La0.75Ti3O12 (BLT), have been stimulated by prospective technical applications in ferroelectric nonvolatile memories. See, e.g., Arauio et al., xe2x80x9cFatigue-free ferroelectric capacitors with platinum electrodes,xe2x80x9d Nature 374, 627 (1995); Park et al., xe2x80x9cLanthanum-substituted bismuth titanate for use in non-volatile memories,xe2x80x9d Nature 401, 682 (1999). This is due in large part to the high fatigue endurance of SBT and other layered perovskite materials. However, the application of such polycrystalline perovskite materials suffers from certain limitations. For instance, it is difficult to obtain ferroelectric properties that are homogeneous over the different cells of a large capacitor array when the lateral size of the ferroelectric cells drops below 100 nm (corresponding to cell sizes needed for Gigabit memories). See Gruverman, xe2x80x9cScaling effect on statistical behavior of switching parameters of ferroelectric capacitors,xe2x80x9d Appl. Phys. Lett. 75,1452 (1999). It is believed that the use of epitaxial films is a way to overcome this non-uniformity problem of ferroelectric properties. See Kingon, xe2x80x9cMemories are made of . . . ,xe2x80x9d Nature 401,658(1999). Moreover, the existence of ferroelectric properties and their dependence on the cell size and material in such small structures (i.e., size effect) has been recently addressed in Alexe et al., xe2x80x9cPatterning and switching of nanosize ferroelectric memory cells,xe2x80x9d Appl. Phys. Lett. 75, 1793 (1999); and J. F. Scott: Abstracts of 12th International Symposium on Integrated Ferroelectrics (xe2x80x9cNano-Scale Ferroelectrics for Gbit Memory Applicationxe2x80x9d), Aachen, Germany, Mar. 12-15, 2000, p. 102.
Successful efforts in the epitaxial growth of SBT thin films deposited by pulsed laser deposition (PLD), metalorganic chemical vapor deposition (MOCVD), and RF magnetron sputtering methods, have been reported in Lettieri et al., xe2x80x9cEpitaxial growth of (001)-oriented and (110)-oriented SrBi2Ta2O9 thin films,xe2x80x9d Appl. Phys. Lett. 73, 2923 (1998); and Pignolet et al., xe2x80x9cOrientation dependence of ferroelectricity in pulsed-laser-deposited epitaxial bismuth-layered perovskite thin films,xe2x80x9d Appl. Phys. A70, 283 (2000). In all of these works, special single crystalline substrates such as SrTiO3, LaAlO3-Sr2AlTaO6, LaSrAlO4, and MgO of various orientations have been used to grow epitaxial c-axis-oriented as well as non-c axis oriented SBT thin films. It was generally found that c-axis-oriented epitaxial SBT films (i.e., films with their (001) plane parallel to the substrate surface) can be grown on SrTiO3 (100) substrates, whereas epitaxial SBT films that have the (116) and (103) plane parallel to the substrate surface, grow on SrTiO3(110) and (111) substrates, respectively. In Lettieri et al., xe2x80x9cEpitaxial growth of non-c-oriented SrBi2Nb2O9 on (111) SrTiO3,xe2x80x9d Appl. Phys. Lett. 76, 2937 (2000), properly oriented ferroelectric films were epitaxially grown on SrTiO3. Specifically, it was reported that heterostructures consisting of an underlying (111) SrRuO3 epitaxial electrode and an epitaxial (103) SrBi2Nb2O9 overlayer were prepared, as SrRuO3 is closely lattice matched with SrTiO3 and chemically compatible with both SrBi2Nb2O9 and SrTiO3.
The above observations are, however, not of high practical significance for memory devices, because SrTiO3 crystals are not suitable substrates in microelectronics. For a better compatibility with silicon-based microelectronics, epitaxial SBT films should be grown on silicon substrates. The epitaxial growth of non-c-axis-oriented SBT on Si(100) has not heretofore been reported. As a general matter, it is widely acknowledged that the integration of complex feroelectric materials with silicon-based devices has been elusive to date. See, e.g., Kingon, xe2x80x9cMemories are made of . . . ,xe2x80x9d Nature 401, 658 (1999).
The growth of non-c-oriented bismuth-containing ferroelectric films having a layered perovskite structure, such as SrBi2Ta2O9 (SBT), SrBi2Nb2O9 (SBN) and SrBi2(Ta,Nb)2O9 (SBTN), is of particular significance because the vector of the spontaneous electrical polarization in these layered perovskite materials is directed along the a-axis. By contrast, a c-oriented layered perovskite material does not have a polarization component along its film normal (perpendicular to the film plane). However, if the layered perovskite material is to be used in a ferroelectric thin-film capacitor with electrodes on the top and bottom film surfaces as in the geometry used for dynamic random access memory, a normally oriented polarization component is essential. It would therefore be desirable to grow non-c-axis-oriented layered perovskite materials.
One example of a c-axis oriented silicon/metal oxide heterostructure that, for the purposes of the present invention, is not desirable, is disclosed in U.S. Pat. No. 5,270,298. Specifically, a buffer layer of yttria-stabilized zirconia is grown on a silicon substrate. A template of a c-axis oriented anisotropic perovskite material, such as bismuth titanate (Bi4Ti3O12) is grown on the buffer layer. A cubic metal oxide such as a perovskite material of highly-oriented crystallinity is then able to be grown on the template layer. In the example provided in this patent, the metal oxide is Pb1-yLayZr1-xTixO3 (PLZT), where 0 less than x less than 1 and 0 less than y less than 1.
Epitaxial SrRuO3 thin films, have been found useful as electrodes for ferroelectric capacitors, due to the high thermal and chemical stability of SrRuO3 and because of its good lattice match with SrTiO3 and Pb(ZrTi)O3. SrRuO3 is a pseudocubic perovskite with a slight orthorhombic distortion due to the tilting of the RuO6 octahedra. High-quality epitaxial SrRuO3 films have been successfully deposited on different substrates, such as SrTiO3(100) and LaAlO3(100) and by different methods like off-axis sputtering and PLD, as reported in Eom et al., xe2x80x9cSingle-Crystal Epitaxial Thin Films of the Isotropic Metallic Oxides Sr1-xCaxRuO3 (0xe2x89xa6x xe2x89xa61),xe2x80x9d Science 258,1766 (1992); Chen et al., xe2x80x9cEpitaxial SrRuO3 thin films on (001) SrTiO3,xe2x80x9d Appl. Phys. Lett. 71, 1047 (1997); and Zakharov et al., xe2x80x9cSubstrate temperature dependence of structure and resistivity of SrRuO3 thin films grown by pulsed laser deporition on (100) SrTiO3,xe2x80x9d J. Mater. Res. 14, 4385 (1999).
Recently, epitaxial (116)- and (103)-oriented SBT thin films grown on SrRuO3 base electrodes deposited on lattice-matched perovskite SrTiO3 substrates have been demonstrated by Ishikawa et al., xe2x80x9cElectrical properties of (001)- and (116)-oriented epitaxial SrBi2Ta2O9 thin films prepared by metalorganic chemical vapor deposition,xe2x80x9d Appl. Phys. Lett. 75, 1970 (1999); Zurbuchen et al.: Abstracts of 12th International Symposium on Integrated Ferroelectrics (xe2x80x9cMorphology and Electrical Properties of Epitiaxial SrBi2Ta2O9 Filmsxe2x80x9d), Aachen, Germany, Mar. 12-15, 2000, p. 51; Saito et al.: Abstracts of 12th International Symposium on Integrated Ferroelectrics (xe2x80x9cCharacterization of Residual Stress Free (001)- and (116)-oriented SrBi2Ta2O9 Thin Films Epitaxially Grown on (001) and (110) SrTiO3 Single Crystalsxe2x80x9d), Aachen, Germany, Mar. 12-15, 2000, p. 71; Piqnolet et al., Abstracts of 12th International Symposium on Integrated Ferroelectrics (xe2x80x9cDependence of Ferroelectricity in Epitaxial Pulsed Laser Deposited Bismuth-Layered Perovskite Thin Films on the Crystallographic Orientationxe2x80x9d), Aachen, Germany, Mar. 12-15, 2000, p.111; and Lettieri et al., xe2x80x9cEpitaxial growth of non-c-oriented SrBi2Nb2O9 on (111) SrTiO3,xe2x80x9d Appl. Phys. Lett. 76, 2937 (2000). However, as discussed above, these substrates are not suitable for use in the fabrication of integrated devices for microelectronic applications.
According to one embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising silicon, a buffer layer formed on the substrate, and a non-c-axis-oriented, electrically-conductive template layer formed on the buffer layer. The template layer comprises a perovskite oxide compound. A non-c-axis-oriented, anisotropic perovskite ferroelectric layer is formed on the template layer.
According to another embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising (100)-oriented silicon, a buffer layer comprising (100)-oriented, yttria-stabilized zirconia formed on the substrate, an electrically-conductive template layer formed on the buffer layer in which the template layer comprises a (110)-oriented perovskite oxide compound, and a (116)-oriented, layered perovskite ferroelectric film formed on the template layer.
According to yet another embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising (100)-oriented silicon, a buffer layer comprising (100)-oriented, yttria-stabilized zirconia formed on the substrate a buffer layer comprising (111)-oriented magnesium oxide formed on the yttria-stabilized zirconia, an electrically-conductive template layer formed on the buffer layer in which the template layer comprises a (111)-oriented perovskite oxide compound, and a (103)-oriented, layered perovskite ferroelectric film formed on the template layer.
According to still another embodiment of the present invention, an epitaxial, Aurivillius-phase metal oxide/crystalline silicon heterostructure structure is provided in which the Aurivillius-phase metal oxide has a crystallographic c-axis. The structure comprises a crystalline silicon substrate having a (100) orientation, a buffer layer structure epitaxially oriented upon the crystalline silicon substrate, and an electrically conducting epitaxial template layer. The electrically conducting epitaxial template layer comprises an anisotropic perovskite that is formed epitaxially on the epitaxial buffer layer structure. The buffer layer structure causes the epitaxial template layer to be grown with a substantially non-c-axis orientation. The epitaxial, Aurivillius-phase metal oxide layer is formed upon the epitaxial template layer and has a substantially non-c axis orientation. The epitaxial, Aurivillius-phase metal oxide layer has an electrical polarization almost or exactly perpendicular to the crystallographic c-axis. The electrical polarization thus has a non-zero component perpendicular to the epitaxial, Aurivillius-phase metal oxide layer.
According to an additional aspect of the present invention, a method is provided for epitaxially growing a layer of a non-c-axis oriented ferroelectric material. In this method, a non-c-axis oriented, electrically conductive template layer comprising a perovskite oxide compound is formed on a zirconia-buffered, silicon-containing substrate. A non-c-axis oriented ferroelectric material is grown on the template layer.
The growth of epitaxial films in one of the non-c-axis orientations, for example (118) and (104) for BLT or (116) and (103) for SBT, with the (major) spontaneous polarization vector being inclined to the film plane, was originally disclosed in copending, commonly assigned U.S. patent application Ser. No. 09/875,630 filed Jun. 6, 2001, the contents of which are incorporated herein by reference in its entirety. In accordance with the present disclosure, it is believed that the best choice would be (100)-oriented (xe2x80x9ca-axis-orientedxe2x80x9d) epitaxial films, because in this case the (major) polarization vector would be perpendicular to the film plane and result in a maximum value of the remanent polarization. Such a-axis-oriented films have heretofore been grown only on complex-oxide single-crystal substrates that are not suitable for use in the fabrication of integrated devices for microelectronic applications. To enable the integration of bismuth-layered perovskite films into silicon-based microelectronics, the growth of uniformly a-axis-oriented epitaxial films on electroded Si substrates is necessary and has been a long-standing challenge so far.
Accordingly, the present invention successfully demonstrates the epitaxial growth of a-axis-oriented ferroelectric thin films on substrates suitable for integrated microelectronic devices, such as Si(100) substrates, covered with a very thin, strained (110)-oriented electrode layer, such as SrRuO3, on a buffer layer such as (100)-oriented yttria-stabilized zirconia (YSZ). In specific examples, the orientation of BLT films turned out to critically depend on the thickness of the (110)-oriented SrRuO3 electrode layer, with (100) and (118) BLT orientations competing. A comprehensive variation of the SrRuO3 electrode layer thickness and of other deposition conditions enabled the growth of uniformly a-axis-oriented BLT films on Si(100).
Therefore, according to another embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising a (100)-oriented material, a buffer layer formed on the substrate and having a (100) orientation, a template layer formed on the buffer layer and comprising a (110)-oriented perovskite oxide compound, and an epitaxially a-axis-oriented ferroelectric layer formed on the template layer. The ferroelectric layer exhibits a vector of spontaneous polarization oriented substantially normal to a plane of the ferroelectric layer.
According to yet another embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising a (100)-oriented material, a buffer layer formed on the substrate and comprising YSZ (100), a template layer formed on the buffer layer and comprising a (110)-oriented perovskite oxide compound, and an epitaxially a-axis-oriented ferroelectric layer comprising Bi4Ti3O12 formed on the template layer. The ferroelectric layer exhibits a vector of spontaneous polarization oriented substantially normal to a plane of the ferroelectric layer.
According to still another embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising a (100)-oriented material, a buffer layer formed on the substrate and comprising YSZ (100), a template layer formed on the buffer layer and comprising a (110)-oriented perovskite oxide compound, and an epitaxially a-axis-oriented ferroelectric layer comprising SrBi2Ti2O9 the ferroelectric layer formed on the template layer. The ferroelectric layer exhibits a vector of spontaneous polarization oriented substantially normal to a plane of the ferroelectric layer.
The present invention also provides a method for epitaxially growing a layer of a-axis oriented ferroelectric material that comprises the following steps. A template layer is formed by depositing a (110)-oriented perovskite oxide compound on a (100)-oriented substrate buffered with a (100)-oriented buffer layer. An epitaxially a-axis oriented ferroelectric layer is grown on the template layer. The ferroelectric layer so grown exhibits a vector of spontaneous polarization oriented substantially normal to a plane of the ferroelectric layer.
Certain process conditions are found to be preferable in successfully growing the a-axis ferroelectric layer having the advantageous characteristics described herein. First, the perovskite oxide compound, which preferably comprises strontium ruthenate or lanthanum nickelate, is deposited to a thickness of approximately 10 to approximately 15 nm, thereby in effect stretching this template layer quite thin to optimize its use as a template layer for the growth of (100)-oriented nuclei. Second, the material of the ferroelectric layer is deposited at a rate of approximately 100 to approximately 200 nm/min to promote the growth of (100)-oriented nuclei. Third, the oxygen pressure is maintained at approximately 0.9 mbar to approximately 5 mbar during growth of the ferroelectric layer to suppress re-evaporation of bismuth-containing species.
The present invention further provides non-c-axis, including a-axis, oriented ferroelectric structures produced according to the methods summarized and described herein.
It is therefore an object of the present invention to provide a structure, and a method for growing the same, that contains a non-c-axis oriented ferroelectric film, which structure exhibits properties useful in the development of data storage devices.
It is a further object of the present invention to provide a template on a buffered silicon substrate suitable for the growth of a non-c-axis oriented ferroelectric film.
Some of the objects of the invention having been stated hereinabove, other objects will become evident as the description proceeds when taken in connection with the accompanying drawings as best described hereinbelow.